A silicon controlled rectifier, referred to by its family name "thyristor" herein, can be thought of as a switchable diode controlled by a third terminal, the gate. If a supply voltage is applied between the anode and cathode of the thyristor, and the supply voltage is less than a critical value, the breakover voltage, and no trigger current is applied to the gate, the thyristor will remain off. When a large enough trigger current or voltage is applied to the gate, the forward breakover voltage of the thyristor will decrease to a value approximating the forward conduction drop of a diode, and the thyristor will turn on conducting as a diode. Once on, the thyristor latches on regardless of the voltage or current applied to the gate. For the thyristor to be turned off, the anode to cathode current must be reduced to near zero value by external means. In a phase-controlled thyristor, this occurs either by load current being reduced to zero by AC line voltage excursions or, in the case of continuous conduction, by the firing of another thyristor in the same bridge which contains the conducting thyristor.
Two thyristors are used for control of bipolar currents to a load. The two thyristors are connected in a back-to-back configuration with the anode of each thyristor coupled to the cathode of the other thyristor.
Circuits using back-to-back thyristor bridges include circuits for DC motor regulation, and circuits for DC bus voltage control in AC motor speed controllers, both of the type capable of power transfer from the motor back to the AC lines (regenerative controllers). The thyristor converters in such circuits typically consist of two back-to-back thyristor bridges each having a plurality of thyristor pairs connected anode-to-cathode and cathode-to-anode. A fault which effectively shorts two AC lines in such circuits can occur if certain thyristors in one bridge are turned on while any thyristor in the opposite bridge is conducting. Such a short circuit can damage or destroy the conducting thyristors.
Prior art protection circuits have been designed to detect the end of conduction of a thyristor in one bridge to prevent the firing of a thyristor in the other bridge until such firing is safe. Such prior art circuits utilize load current sensing techniques in conjunction with a time delay of typically a few milliseconds duration. The time delay is necessary, in part, because of the difficulty of determining precisely when current through a thyristor reaches zero magnitude. One disadvantage of such circuits is that they must accurately sense that a current, whose peak value may be in the 1,000 ampere range, has decreased to less than 0.1 ampere. Such resolution is difficult to obtain and is even more difficult to maintain over the life of the circuit. Furthermore, such prior art circuits are sensitive to spurious ringing currents that occur as a thyristor pair turns off. These ringing currents, which are not indicative of thyristor conduction, cause further unnecessary delay in the transfer of control between thyristor bridges; under some conditions, ringing currents may thwart transfer completely.
Consequently, a need has arisen for a protective circuit which is insensitive to ringing, does not drift over time, and precisely detects the end of thyristor conduction thereby avoiding unnecessary delay in the transfer of control from one bridge to another.